Conventionally, a spark plug for an internal combustion engine, for example, automotive ignites an air-fuel mixture through spark discharge. In recent years, high output and low fuel consumption have been demanded from internal combustion engines. To fulfill such requirements, plasma jet spark plugs are used. A plasma jet spark plug provides quick propagation of combustion and exhibits such a high ignition performance as to be capable of reliably igniting even a lean air-fuel mixture having a higher ignition-limit air-fuel ratio.
Such a plasma jet spark plug has a structure in which an insulator formed from ceramics or the like surrounds a spark discharge gap between a center electrode and a ground electrode integrated with a metallic shell, thereby forming a small-volume discharge space called a cavity. A high voltage is applied to the spark discharge gap so as to perform spark discharge. By virtue of associated occurrence of dielectric breakdown, current can be applied at a relatively low voltage. Thus, through transition of a discharge state effected by further supply of energy, plasma is generated within the cavity. Since the ground electrode is located frontward of the insulator having the cavity formed therein, the ground electrode has a hole called an orifice formed therein. Plasma is emitted outward through the orifice, thereby igniting an air fuel mixture.
Meanwhile, if a clearance is present between the ground electrode and the insulator and if, during emission of plasma through the orifice, energy of the plasma leaks into the clearance and into a clearance between the metallic shell and the insulator communicating with the former clearance, energy of plasma emitted through the orifice reduces, thereby causing impairment in ignition performance. To cope with the problem, there has been proposed a plasma jet spark plug in which an insulator (housing) is provided in close contact with a ground electrode (external electrode) so that no clearance is present between the insulator and the ground electrode. For example, Japanese Patent Application Laid-Open (kokai) No. 2006-294257 discloses a plasma jet spark plug wherein, the ground electrode and a metallic shell are integrally formed, so that the ground electrode (a portion of the metallic shell which corresponds to the ground electrode) is accurately positioned in relation to the metallic shell. Accordingly, dimensional adaptation of the insulator will be sufficient for establishment of close contact between the insulator and the ground electrode. When the insulator is retained by the metallic shell, a front end portion of the insulator abuts the ground electrode.
In the process for manufacturing the plasma jet spark plug, since the insulator is retained by the metallic shell through crimping, a displacement of the insulator may arise. However, precise control of the displacement is difficult. In some displaced condition, a crimping load may be applied to the insulator in such a manner that a front end portion of the insulator strongly butts against the ground electrode, potentially causing breakage of the insulator.